The commercial broadcast industry and businesses which advertise through the RF broadcast media need to know the sizes of the audiences which are tuned to particular stations at particular times. This need has been met primarily through the use of verbal or written audience participation surveys. With respect to radio, a majority of the listening occurs in automobiles. However, a problem with written surveys is that listeners cannot practically make a record of their listening tendencies while driving.
In order to make a record of listening tendencies while driving, passive electronic RF monitoring equipment has been used to remotely identify the stations to which tuners may be tuned. Generally speaking, audiences' tuners use predetermined signals, such as local oscillator signals, that are related to the frequencies of the respective stations currently being tuned in. The local oscillator signals are broadcast or otherwise emitted from the tuners as very weak signals that sensitive monitoring equipment can detect.
This remote monitoring technique is desirable because it does not require cooperation from an audience, hence reducing or eliminating a host of inaccuracies and costs associated with audience participation surveys. Furthermore, large sample sizes may be monitored at low cost relative to audience participation survey techniques.
Using survey methodology in a remote monitoring system, a highly desirable goal is to maintain a "level playing field", i.e., all stations have an equal opportunity of being recorded during the survey. When a vehicle is detected passing through a survey zone, no bias or preference should occur in detecting the station on the vehicle's radio over another station, regardless of its frequency.
Prior art conventional remote monitoring systems have failed to adequately address many different situations that lead to skewed or biased survey data toward or against an individual station or groups of stations. This bias, described as station bias herein, is different for differing radio stations. For example, multiple tuners located near one another and tuned to the same station may be indistinguishable from one another by the monitoring equipment so as to bias survey data in favor of less popular stations. In addition, conventional monitoring equipment may fail to identify some radio stations due to a weak local oscillator signal at a particular tuner.
The level of background electronic noise may cause local oscillator signals at some frequencies to be more readily detectable than other frequencies leading to station bias in favor of stations whose related local oscillator signals may have a lower level of background noise. In addition, traffic speed, or unexpected variation in traffic speed, affects the duration over which the local oscillator signals may be detected, thus leading to station bias. Still further, the accuracy of the survey data obtained from conventional equipment may be affected by environmental conditions. Temperature and/or humidity fluctuations affect electronic system monitoring and detecting capability differently along the range of frequencies of local oscillator signals. Hence, local environmental conditions may bias data in favor of some stations and against other stations.
U.S. Pat. No. 5,410,724 discusses a remote radio monitoring system and methodology for obtaining accurate survey data. This system ignores certain detectable and detected data which might otherwise be included in a survey to refrain from introducing unfair biases. This system also attempts to equalize the detection of the noisiest local oscillator signal with the detection of the other less noisy oscillator signals. Furthermore, this system attempts to prevent station bias caused by environmental fluctuations with the proper selection of electronic components.
However, in the system described in U.S. Pat. No. 5,410,724, as well as the other prior art systems, there was no way of obtaining a measure of the accuracy of the survey data to determine if biases exist toward or against individual frequencies or groups of frequencies within the band of broadcast frequencies for the broadcast stations. Furthermore, when a station bias does exist for an individual broadcast station, these systems do not compensate for this station bias.